This invention relates to the formation of substrates which contain a filler such that the filled substrate is rendered electrically conductive. In particular, this invention relates to such a process wherein the conductive filler is formed from discrete particles having a copper or nickel metal surface.
It is frequently desired to incorporate a conductive metal filler in a non-conductive substrate. Such composites are useful for electromagnetic interference (EMI) shielding and so forth, as indicated in Encyclopedia of Chemical Technology, Kirk-Othmer, III Ed., Volume 18, pp. 784-8.
The electrical conductivity occurs by interparticle contact of the filler species. Therefore, it is necessary that the exposed surface of the filler species is highly conductive. Readily available powder of conductive metals such as nickel and copper, however, comprises a non-conductive oxide content present at the surface of the particles. Commercial nickel or copper powders show no or unsufficient conductivity for direct compounding into substrates such as engineering plastics. Therefore, the practice of metal filler is mostly confined to less oxidation-sensitive but more expensive metals such as silver.
It is possible to remove the copper oxide or nickel oxide surface layer by acidic treatment prior to compounding with the plastics. However, this introduces a quite cumbersome additional processing step, since the fine metal particles are readily re-oxidized. Moreover, composites of thermoplastics and acid washed or freshly prepared copper or nickel particles may initially show a good conductivity but tend to loose this conductivity upon aging. This problem of deteriorating conductivity has been recognized in the patent literature and some suggestions were made to overcome the problem.
Thus, Japanese laid-open patent application no. 127,742/83 mentions the use of reducing phosphorous acid derivatives resulting in initial conductivity of the composites. However, it was found that the trivalent phosphorous acid compounds do not remain stable for long periods and fail to produce composites having long-term conductivity. Therefore, a specific pentavalent phosphorus compound was suggested.
British patent application no. 2,171,410 discloses the deposition of an amine compound onto metal powder prior to incorporation in a group of specific polymers containing carboxyl groups. For filling other more general polymers the further deposition of a silane coupling agent is required. The Examples of this patent application essentially are directed to amine-deposited magnetic iron powder, where the surface condition of the metal powder is less important, but rather bulk corrosion of the iron powder should be avoided.
European patent application no. 221434 discloses the improvement of the electrical conductivity of plastic materials by dispersing highly electrically conductive filler elements throughout the volume of the plastic coupled with dispersion of low ionization potential materials in the space separating the conductive fillers. Suitable low ionization potential materials include tertiary amines such as dimethyl aniline.
U.S. Pat. No. 4,122,143 discloses a process for producing a conductive cured product comprising the step of reducing a cuprous or cupric compound to metallic copper. Suitable reducing agents include inorganic acids, such as phosphorous acid, trivalent phosphorous-containing acid or ester type compounds such as phosphites, and reducing sugars such as glucose.
Applicants' co-pending application U.S. Ser. No. 068,593/EP-A-297677 deals with the problem of forming a layer of conductive metal on the surface of a substrate. In contrast, the instant invention deals with the problem or rendering an entire three dimensional substrate conductive.